RNA viruses whose genome is composed of a single RNA strand capable of replication in the cytoplasm of a host by direct RNA replication are widespread, many varieties of which are known to infect plants. Such viruses are sometimes termed "(+) strand RNA viruses" since the infective RNA strand, that normally found encapsidated in the virus particle, is a messenger-sense strand, capable of being directly translated, and also capable of being replicated under the proper conditions by a direct process of RNA replication. Viruses belonging to this group include "single component (+) strand RNA viruses", which replicate in the absence of trans-acting viral replication elements. These viruses may include, but are not limited to any of the representatives of the following virus groups, Carlavirus, Closteroviridae, Luteoviridae, Potexvirus, Potyviridae, Tombusviridae, Tobamovirus and Tymovinis. (Similar viruses, which in the host cell produce a trans-acting replication element, are not included in this group.) In these cases, the entire virus genome is contained within a single RNA molecule, while in the multicomponent RNA plant viruses, the total genome of the virus consists of two or more distinct RNA segments, each separately encapsidated. For general review, see General Virology, S. Luria and J. Darnell; Plant Virology 2nd ed., R. E. F. Matthews, Academic Press (1981). For a general review of (+) strand RNA replication, see Davies and Hull (1982) J. Gen. Virol. 61:1.
Despite the well-documented diversity between virus groups, recent studies have shown striking similarities between the proteins, which function in RNA replication. Sequence homologies have been reported between the cowpea mosaic virus, poliovirus and foot-and-mouth disease virus, (Franssen, H. (1984) EMBO Journal 3,855). Sequence homologies have been reported between non-structural proteins encoded by alfalfa mosaic virus, brome mosaic virus and tobacco mosaic virus, Haseloff, J. et al. (1984), Proc. Nat. Acad. Sci. USA 81, 4358, and between non-structural proteins encoded by sindbis virus, Ahlquist, P. et al. (1985) J. Virol. 53, 536. Evidence of such substantial homology in proteins related to the replication functions indicate that the viruses share mechanistic similarities in their replication strategies and may actually be evolutionarily related. Ahlquist et al., in U.S. Pat. No. 5,500,360 made modifications to the genomic RNA of a (+) strand RNA virus of a multipartite Brome mosaic virus. The modified RNA was used to transfer a desired RNA segment into a targeted host plant protoplast, and to replicate that segment and express its function within the host protoplast.
In contrast to the Brome mosaic virus (BMV), the tobacco mosaic virus (TMV) is one member of a class of plant viruses characterized by a single RNA genome. The genetic material of the virus is RNA, and the total genetic information required for replication and productive infection is contained in one discrete RNA molecule. Infection of a host plant cell occurs when the single RNA component of the viral genome has infected the cell, for example by exposing a plant to a virus preparation. Infection may also be achieved by exposing a plant cell or protoplast to a virus preparation. TMV does not require coat protein for infection. The RNA component is both necessary and sufficient for replication and productive infection. The TMV genome is a single messenger-sense RNA. The term "messenger-sense" denotes that the viral RNAs can be directly translated to yield viral proteins, without the need for an intervening transcription step.
Complete cDNA copies of the genetic component of TMV have been cloned. Construction of a library of subgenomic cDNA clones of TMV has been described in Dawson et al., Proc. Natl. Acad. Sci. USA 83:1832-1836 (1986) and Ahlquist et al., Proc. Natl. Acad. Sci. USA 81:7066-7070 (1984). Several examples of TMV transcription vectors are described below. DNA from each of the TMV cDNA-containing plasmids can be cleaved. The linear DNA thus produced can be transcribed in vitro in a reaction catalyzed by RNA polymerase. A T7 promoter in the transcription vector allows RNA synthesis to initiate at the 5' terminus of each TMV sequence, and transcription continues to the end of the DNA template. The 5' terminus of tobacco mosaic virus (TMV) RNA, was identified as m.sup.7 G.sup.5' ppp.sup.5' Gp. Zimmern, D., Nucleic Acid Res. 2:1189-1201 (1975). Keith, J. and fraenkel-Conrat, H. FEBS Lett. 57:31-33 (1975). Ahlquist, U.S. Pat. No. 5,500,360, working with Brome mosaic virus, reported that when transcription is carried out in the presence of a synthetic cap structure, m.sup.7 GpppG, as described by Contreras, R., et al. Nucleic Acids Res. 10:6353, (1982), RNA transcripts are produced with the same capped 5' ends as authentic BMV RNAs. Ahlquist concluded that these RNAs are active messengers in in vitro translation systems and direct production of proteins with the same electrophoretic mobilities as those translated from authentic BMV RNAs. However, Ahlquist found that, "if the cap analog was omitted during in vitro transcription, no infection was detected, even if inoculum concentration was increased 20-fold." Further, Ahlquist taught only a viral vector having "no extraneous nonviral sequences between the cap and the 5' terminus of the viral sequence." In Ahlquist's work on BMV, U.S. Pat. No. 5,500,360, a transcription vector was employed which preserved the exact 5' terminal nucleotide sequence of viral RNA. It is now generally accepted that capping is necessary for infectivity and that no intervening sequence can be present between the cap and the 5' terminus of the viral sequence.
The work of Ahlquist leaves us with difficult problems to overcome if we are to obtain a workable viral vector or a commercially viable viral vector. One such problem is the cost of using capping structures and cap analogs. Another such problem is that multipartite viral vectors are difficult to use relative to a single component viral vector. Multipartite viruses require more than one unit to infect and achieve replication in a host plant, and multipartite viruses require a trans acting replication element to achieve replication. No one has yet found a way to unite the multiple strands of a multipartite virus into an RNA molecule comprising the entire genome of a (+) strand RNA virus as suggested and claimed by Ahlquist.
Therefore, there is a need for a viral vector that can accept an intervening base or intervening sequence of bases between the cap and the 5' terminus of the viral sequence and undergo transcription and replication. There is also a need for a viral vector that can undergo transcription and replication in the absence of a capping structure.
Here we teach solutions to the problem by demonstrating:
1. Infection of a host and replication of a viral vector in vivo in the presence of a base or a sequence of bases placed 5' to the origin of replication in the absence of a capping structure or cap analog. PA1 2. Infection of a host and replication of a viral vector in vivo in the absence of a capping structure or a cap analog, and in the absence of a base or a sequence of bases placed 5' to the origin of replication. PA1 3. Infection of a host and replication of a viral vector in vivo in the presence of an intervening base or an intervening sequence of bases placed 5' to the origin of replication and in the presence of a capping structure or cap analog. PA1 forming a full-length cDNA transcript of the RNA virus; PA1 cloning the cDNA in a transcription vector; PA1 modifying the cDNA by inserting a non-viral DNA segment in a region able to tolerate such insertion without disrupting RNA replication thereof; PA1 transcribing the modified cDNA corresponding to the RNA component of the single component virus; PA1 infecting virus-susceptible protoplasts, cells, tissues or whole plants with transcribed RNA, either in solution or encapsidated, of the modified RNA comprising messenger-sense RNA containing an exogenous RNA segment.
The viral vectors demonstrated here have utility in discovery the function of genes, and in production of therapeutic proteins.